1. Field of the Invention
The present invention relates to novel photopolymerizable compositions for image formation.
2. Description of the Prior Art
Photopolymerizable compositions are useful for image formation. In turn the images are used as reliefs for printing plates or pattern plates to prepare papier-mache molds (hereinafter referred to as a matrix) or as plates for dry-offset printing and the like. The photopolymerizable compositions so used should be characterized by having an image forming portion which is rapidly photopolymerized under a light image, and a non-image forming portion which can be easily removed by washing with water, an aqueous solution or some other solvent. The image reproducibility of the compositions is high, and the image forming portion has high chemical resistance and mechanical properties. When such a relief is used as the printing plate, it is necessary that the printing plate prepared by using a photopolymerizable composition have high strength and wear resistance and high printing resistance, whereby the fine lines and fine half-tone dots of the printing plate should not broaden and wear-out as a result of printing. When the relief is used as a pattern plate, it is necessary that the pattern plate have sufficient hardness, strength and water resistance whereby the fine lines and fine half-tone dots of the printing plate prepared by molding lead in the matrix prepared in turn by using the pattern plate should not be broadened and worn-out by repeating the preparation of the matrix.
The conventional photopolymerizable compositions which can be photopolymerized by light-irradiation include an unsaturated polyester resin, a vinyl monomer and a photosensitizer. The resin compositions can be used for image forming compositions. The unsaturated polyesters included in the resin compositions can be produced by the polycondensation of a polyol with an unsaturated carboxylic acid or an ester thereof such as maleic acid, maleic anhydride, dimethyl maleate, diethyl maleate, fumaric acid, dimethyl furmarate, citraconic acid, citraconic anhydride, itaconic acid, itaconic anhydride, dimethyl itaconate and the like. Among these unsaturated polyesters, unsaturated polyesters produced by the polycondensation of a polyol with maleic acid (hereinafter referred to as maleic acid type unsaturated polyesters) have special polymerizable characteristics and the selection of the vinyl monomer component is limited. The resin compositions which contain styrene or a styrene derivative as the vinyl monomer component have relatively high photopolymerizing properties when exposed to light, however, the resulting photopolymerized products posssess inferior mechanical strength. Accordingly, it is hard to use these resin compositions as a photopolymerizable composition for image formation.
The unsaturated polyesters produced by the polycondensation of a polyol with itaconic acid (hereinafter referred to as itaconic acid type unsaturated polyesters) are easily colored in production which decreases the photopolymerizability of the composition when exposed to light. Thermal polymerization of the unsaturated group in the polyester results during the production of the itaconic acid type unsaturated polyesters and it is difficult to produce unsaturated polyesters having desirable characteristics. A need, therefore, continues to exist for a photopolymerizable composition which is rapidly photopolymerized when exposed to light and which results in an image which has good chemical resistance and mechanical properties.